1. Field of the Invention
The present invention relates generally to methods of making metallic carbide powders and, more particularly, to methods of making metallic carbide powders having a particle size of greater than 0.0 micrometers, but less than 0.2 micrometers (.mu.m).
2. Description of the Related Art
Various metallic carbides, such as tungsten carbide, are useful for making articles which require substantial mechanical strength, such as dies, cutting tools, and drilling tools. One method for making tungsten carbide has been disclosed in the article, "The Direct Production of WC from WO.sub.3 by Using Two Rotary Carburizing Furnaces", Journal of Japan Society of Powder and Powder Metallurgy, Volume 26, No. 3, pages 90+, by M. Miyake, et al. The article discloses a method of forming WC by pelletizing and reacting a WO.sub.3 and carbon mix in a series of two rotary furnaces. The first furnace operates in nitrogen and allows the reaction to form W, W.sub.2 C, WC, and C which are then fed directly into a second rotary furnace operating in hydrogen for final carburization. The method is difficult because in order to maintain tight control of the carbon content in the final product, it requires precise control of the CO/CO.sub.2 partial pressure ratio.
Another method of making monotungsten carbide (WC) is disclosed in U.S. Pat. No. 5,166,103 to Krstic. The disclosed method entails reacting a mixture of tungsten oxide and carbon powders under vacuum while agitating to enhance the release of carbon monoxide. However, the Krstic method does not lend itself well for large-scale production processes, as it is not desirable to operate large-scale production processes under vacuum.
A third method of making WC powder is disclosed in U.S. Pat. No. 4,664,899 to Kimmel, et al. The disclosed method entails (a) mixing tungsten oxide or ammonium paratungstate with carbon to form a mixture which is substantially reduced in a nonreducing atmosphere in the presence of sufficient carbon to produce a carbon content in the resulting mixture of less than seven percent by weight (wt %) and (b) adding sufficient carbon to the resulting reduced mixture to increase the carbon content to at least the stoichiometric amount needed to form WC prior to carburization of the mixture to monotungsten carbide.
In Kimmel, et al.'s examples, the initial reactant mixture is loaded into boats and stoked through a 1121.degree. C. furnace for 2.5 hours, after which it is necessary to blend the reacted mixture for homogeneity. The final WC products prepared in the Examples had particle sizes, as measured by Fisher Subsieve Size of from 1.00 to 1.38 .mu.m.
In spite of the available methods for making metallic carbides, there remains a need for a non-vacuum, economical process suitable for large-scale production of submicrometer metallic carbides having low free carbon and oxygen contents. It would also be advantageous to have available a method which is capable of making a mixture of at least two types of metallic carbides.